W-CDMA (Wideband Code Division Multiple Access) is a radio interface for IMT-2000 (International Mobile Communication), which was standardized for use as the 3rd generation wireless mobile telecommunication system. It provides a variety of services such as voice services and multimedia mobile communication services in a flexible and efficient way. The standardization bodies in Japan, Europe, USA, and other countries have jointly organized a project called the 3rd Generation Partnership Project (3GPP) to produce common radio interface specifications for W-CDMA.
The standardized European version of IMT-2000 is commonly called UMTS (Universal Mobile Telecommunication System). The first release of the specification of UMTS has been published in 1999 (Release 99). In the mean time several improvements to the standard have been standardized by the 3GPP in Release 4 and Release 5 and discussion on further improvements is ongoing under the scope of Release 6.
Another issue becoming more and more important for future mobile communication networks is the interworking between different access network technologies in heterogeneous networks.
It has been recognised that WLANs (Wireless Local Area Networks) are, and will continue to be deployed by independent operators and that these WLANs may or may not be interworked with a mobile communication systems, such as 3GPP systems. Further, these WLANs may overlap partially or completely with WLANs that are interworked with Home and/or Visited mobile communication systems. Lastly, WLANs interworked with Home and Visited 3GPP systems may also overlap each other, as well as the radio access networks for each mobile communication system. These situations create multiple permutations of coverage areas and service states which will need to be carefully understood and managed.
In this respect the 3GPP has standardized the interworking of 3G networks and WLAN (see 3GPP TR 22.934: “Feasibility study on 3GPP system to Wireless Local Area Network (WLAN) interworking”, 3GPP TS 22.234: “Requirements on 3GPP system to Wireless Local Area Network (WLAN) interworking” and 3GPP TS 23.234: “3GPP system to Wireless Local Area Network (WLAN) interworking; System Description”, all available at http://www.3gpp.org).
There are number of different possible operating environments where interworking of the 3GPP system and the WLANs may be desired. The 3GPP operates universally in Public, Corporate, or Residential environments. WLANs may also be deployed in any of these environments and it would be advantageous if the standards for 3GPP to WLAN interworking could accommodate all of these environments. Such capability would further enhance the ease of use for the mobile system user and virtually extend the effective coverage areas of each system.
The different environments may involve different administrative domains and wide diversity of WLAN technical capabilities. As an example, the security capabilities and policies may differ between public, corporate and residential WLANs. These differences may lead to different interworking methods between 3GPP and WLANs.
Different scenarios are identified describing different levels of interworking. E.g. scenario 2 describes 3GPP-based access control and charging, scenario 3 relates to accessing 3GPP packet switched (PS) based services, e.g. IP Multimedia Subsystem (IMS), scenario 4 and 5 consider the access to 3GPP PS based services with service continuity.
The protocol details are separated in several documents. In 3GPP TS 24.234: “3GPP system to Wireless Local Area Network (WLAN) interworking; UE to Network protocols” the details of the protocols between WLAN UE and the network are described. In 3GPP TS 29.234: “3GPP system to Wireless Local Area Network (WLAN) interworking; Stage 3 Description” protocols for several network reference points are specified and in 3GPP TS 33.234: “Wireless Local Area Network (WLAN) interworking security” the security architecture, i.e. trust model and security requirements for the interworking of the 3GPP System and WLAN Access Networks are discussed.
The details of the WLAN Access Authentication and Authorization procedure from the architectural view are described in chapter 7.2 of 3GPP TS 23.234 and the W-APN (WLAN Access Point Name) Resolution and Tunnel Establishment in chapter 7.9 of 3GPP TS 23.234.
The description of the GPRS architecture with its entities and functionalities can be found in 3GPP TS 23.060: “General Packet Radio Service (GPRS); Service Descriptions; Stage 2”. Especially the logical architecture in chapter 5.4, the GPRS attach procedure in chapter 6.5, the mobility management states in chapter 6.1.2, the location management procedures in chapter 6.9.2, the Packet Data Protocol (PDP) context activation procedure in chapter 9.2 and the appropriate HLR GPRS subscription data, MM and PDP contexts in chapter 13 are explained.
If a terminal hands over from a 3G network to WLAN and is afterwards in idle mode in 3G network. The terminal has to send periodic Routing Area Update (RAU) messages over the 3G air interface to preserve the MM and PDP contexts in the 3G Serving GPRS Support Node (SGSN). This requires the availability of 3G network coverage and a periodic 3G connection establishment for performing the RAU. If 3G network coverage is not available (user is in a shopping centre, at airport, in the subway) and the routing area (RA) update timer expires, the MM and PDP contexts are deleted in the SGSN. Further the 3G network connection establishment for performing the RAU would increase 3G signalling overhead and User Equipment (UE) power consumption.
In addition, if the terminal starts a 3G service from WLAN coverage, there is no PDP context established in the appropriate 3G SGSN. Thus if handover from WLAN to 3G is initiated the 3G connection for the service must be initiated from scratch by performing and GPRS Attach procedure and a PDP Context Activation which are time consuming and imply a significant delay for service delivery through the 3G network.